Friday the 13th may be considered unlucky, but it's also more common than you might think. In fact, in the modern Gregorian calendar, it's more likely that the 13th day in any month will be a Friday than any other day of the week.

DataGenetics provides a lengthy analysis of the modern calendar to show how the 13th so frequently ends up on a Friday. Due to the configuration of the calendar and leap years, the calendar repeats every 28 years, and within that cycle, Friday the 13th pops up with slightly greater frequency than the 13th does on other days of the week:

In the Gregorian system, the year 2000 was a leap year, and then a 28 year rotation cycle of will pass, and another, and another, but after the third regular cycle rotation/sharing of days, we hit an irregularity at 2100 which is not a leap year. This causes the previous 12 calendars to repeat, and the pattern is not resumed until 12 years later (sort of like stretching that 28 year period over 40 years). The 40 year periods disrupt the distribution symmetry of days.

January 1st, 2000 started on a Saturday, and this dictates the positions of all the other days, and when Friday's [sic] occur. The ratchet of the congruence means some days are more 'lucky' than others before the reset!

It's ultimately a rather small difference in frequency, but there are more charts and data regarding Friday the 13th over at DataGenetics, including the longest period of time that can pass without a Friday the 13th and a spreadsheet logging the 13th days of each month from January 2000 to December 2399.


Friday the 13th [DataGenetics via Neatorama]